Method of employing trivalent ions in bleaching pulp



United States Patent US. Cl. 162-79 9 Claims ABSTRACT OF THE DISCLOSURE The addition of aluminum chloride to either the hypochlorite bleaching stage or the chlorine bleaching stage or both of a pulp bleaching process will increase the quantity of chlorine consumed and this hastens the overall process resulting in a beneficial effect on the brightness of the paper product without an accompanying degradation of the fibers of the pulp.

This invention relates to a method of employing trivalent ions in the bleaching of pulp and more specifically to a method for incorporating compounds of aluminum in the bleaching stages of the pulp to hasten the reaction and thereby reduce the time required for the bleaching stage. The invention produces little deleterious effects upon the fibers of the pulp and achieves a superior brightness or color characteristics of the final paper product.

It is known that commercial cellulose contains impurities which are never completely removed in the pulping process and which impart color to the pulp. Unbleached pulps are unsuited for the manufacture of high grade white papers. The objective in bleaching is the production of a white pulp of stable color obtained at a reasonable cost with a minimum deleterious effect on the physical and chemical properties of the pulp. Two principal reactions take place in bleaching which are the solubilization and removal of the coloring matter and changing of the coloring matter to a colorless form which is as stable as possible to color and thermal energy. Lignin is primarily responsible as far as color is concerned. Cellulose and the hemicelluloses are inherently white. The principal bleaching and purifying agents are chlorine, chlorine dioxide, and hypochlorite although other bleaching agents are known and used.

The amount and type of material removed in bleaching depends upon the type of pulp and the choice of the bleaching process employed. Bleaching of the pulp takes up where the cooking of the pulp leaves off in the purification of wood to a product of the desired color and quality. It may be envisioned that bleaching is a continuation of the cooking process since both processes are concerned with purification. The decision as to the proper distribution of delignification between pulping and bleaching is as much a matter of economics as the properties desired in the finished pulp. At the present time, increased attention is being directed to pulp yield as the cost of the wood for the pulp continues to rise which induces a trend toward integration of cooking and DUUQ Iii-t Li\hl Ul- 3,472,732 Patented Oct. 14, 1969 bleaching, with less purification being performed in the cooking stage and more being performed in the bleach ing stages.

Excessive bleaching is undesirable since it may cause chemical degradation of cellulose and consequently, bleaching conditions must be chosen to hold degradation to a minimum. Bleaching agents differ in their degrading effect on cellulose due to their differences in oxidation. potentials. If bleaching were performed without attack on the cellulose, sheet strength would rise because of the removal of lignin. Under control conditions, pulps can be bleached to a high brightness level without strength loss and in some cases, it has been shown that there may even be a small gain in strength. In many cases, however, sheet strength is decreased by bleaching usually as a result of overbleachin g.

Accordingly, it becomes desirable to hasten or speed up the bleaching reaction so as to avoid degradation of the cellulose. It is known that each commercial pulp has its own particular peculiarities which must be taken into account in bleaching and each pulp should be bleached in accordance with its individual requirements in order to develop the desired qualities without sacrificing other good characteristics of the pulp. Differences in bleaching properties are ascribable to differences in the amount and nature of the encrusting materials. These are a function of the species of wood used, type of cool;- ing agent, and severity of the cooking operation.

Originally, bleaching of wood pulp was performed as a single operation but in recent years, stepwise purification or multistage bleaching has become the common practice. Multistage bleaching is a method of pulp purification whereby the purifying and bleaching chemicals are added in stages, separated by intermediate washing stages with water or alkali, in which the reaction prodnets are removed. In multistage bleaching ,Jhe various impurities in the fiber are removed in a gradual and orderly way without serious degradation df the fiber in any one stage. Multistage bleaching makes bse of the fact that by adding the bleach in stages the soluble reaction products formed in the first stage can be removed by washing before the remaining bleach is added in the second stage. Bleaching in this manner is more etficient because the solubilized materials are removed before they can take up additional chlorine in side reactions. As a result, the bleach demand of the pulp is lowered.

In general, chemical wood pulps are bleached with chlorine compounds, either direct chlorine plus hypochlo rite, hypochlorite alone, or chlorine dioxide. In general, the more stages used in bleaching, the lower the bleach consumption, but the greater the cost of handling. The simplest form of multistage bleaching is a two-stage process wherein part of the chlorine is applied by direct chlorination, followed by a water washing stage, and then a final stage using hypochlorite bleach. However, in bleaching hardwood pulp it is often desirable to prebleach the pulp with sodium hypochlorite under alkaline conditions prior to the chlorination stage. Chlorination stages following the pre-bleaching hypochlorite stage, proceed at a very slow rate unless means are employed, as taught by the present invention, to speed up the re action.

The simple two-stage bleaching procedure of a chlorine HI a l-II I-u bleaching stage followed by a water-washing stage and then a final stage using hypochlorite bleach was initially employed successfully with sulfite pulps. However, with sulfate pulps, it may be desirable to employ an alkali extraction stage with sodium hydroxide between the chlorine and hypochlorite stages to remove the alkalisoluble chlorinated lignin residues.

It will be intuitively clear that a great savings in time can be effected along with a considerable reduction in cost if a means can be found to speed up or hasten the bleaching stages and still prevent degradation of the pulp fibers. The present invention is directed toward that end by teaching that the addition of an aluminum compound to either the hypochlorite bleaching stage or the chlorine bleaching stage or to both stages, will increase the quantity of chlorine (in the form of chlorine or hypochlorite) consumed and thus hasten the reaction and result in a beneficial etfect on the brightness of the final paper product without an accompanying degradation of the fibers of the pulp.

While the invention is applicable to pulps other than to kratt pulps, reference will be had by way of example to the invention as practiced with kraft pulps from which a discussion of the applicability of the invention to other pulps will become apparent to those skilled in this field.

Kraft pulp is pulp which is obtained from the reaction of either coniferous or deciduous wood with a cooking liquor which has as its principal active constituents sodium hydroxide and sodium sulfide. This process is also known as the sulfate process. The reaction is conducted in a closed pressure vessel at a tempeature of approximately 340 F. After the cooking or digestion stage, the pulp is washed to remove residual spent cooking liquor and dissolved wood components. The washed kraft pulp consists primarily of cellulose and hemicellulose along with small amounts of lignin, wood resins, metal salts, and non-cellulose carbohydrate components.

A typical unbleached kraft pulp has a dull-dark brown color. This color is not easily dyed to other colors, nor does it produce a pleasing contrast to printing ink colors. The two alternative methods by which the color of the unbleached kraft pulp may be lightened or brightned are by more severe cooking or digestion, and by bleaching. The present invention is directed to the improvement of color by bleaching under novel conditions.

Accordingly, it is the principal object of the present invention to improve the brightness characteristics of paper products.

It is a further object of the present invention to improve the brightness characteristics of paper products by increasing the efficiency of the bleaching stages of the pulp.

It is a further object of the present invention to increase the speed of reaction of the oxidation bleaching agents employed in the bleaching stages of wood pulp.

It is a further object of the present invention to increase the speed of reaction of the chlorination bleaching stage employed in the bleaching of wood pulp.

It is a further object of the present invention to increase the speed of reaction of the hypochlorite bleaching stage employed in the bleaching of wood pulp.

It is a still further object of the present invention to increase the speed of reaction of the hypochlorite bleaching stage and the chlorine bleaching stage employed in the manufacture of paper products from wood.

As set forth, the present invention relates to an improvement of the bleaching steps of a kraft pulp by hypochlorite or chlorine. A typical unbleached kraft pulp has a dull-dark brown color. Unbleached pulps are unsuited for the manufacture of high-grade white papers. The characteristic dark brown color of the unbleached pulp is not easily dyed to other colors nor does it give a pleasing contrast to printing ink colors. It is known that at least two alternative methods by which the color of the unbleached pulp may be lightened and brightened to result in a higher brightness of the final product is by more severe cooking or digestion and by bleaching. To improve the color by increasing the severity of cooking is extremely costly.

By means of the bleaching processes described herein, the brightness of the pulp, as determined by method T-217 M-48 of the Technical Association of the Pulp and Paper Industry, can be increased by up to 11 points in the hypochlorite stage and up to 7 points in the chlorine stage and the shade of the paper can be made noticeably lighter and more receptive to dyeing.

In regard to the invention as practiced with a hypochlor-ite stage of bleaching, it has been found that the addition of an aluminum compound, such as aluminum chloride, serves to radically increase the quantity of chlorine consumed in a specified time period as compared to the quantity consumed without the addition of the aluminum compound. The resulting increase in brightness of the paper product reaches as high as 19% as measured by a GE. brightness meter over the brightness reached without the addition of the aluminum compound.

With respect to the addition of an aluminum compound to the chlorine or chlorination bleaching stage, the brightness increase is found to approach approximately 9% over that attained without the addition of the aluminum compound to the chlorination bleaching stage.

Perhaps the explanation that can be offered for the effects of the addition of the aluminum compound to the hypochlorite and/or chlorine bleaching stages is that the aluminum cation acts as a catalyst. Preferably, the aluminum compound being composed of a trivalent cation is more effective if combined with a monovalent anion. Generally, in physical chemistry a compound con sisting of a multivalent ion of one charge and a monovalent ion of the opposite charge will affect colloids and may act as a catalyst for some reactions. A compounds catalytic effect increases as the valency difference between the cations and the anions increases.

In order that the beneficial results flowing from the practice of the invention be readily evident, a number of examples and experiments were performed. Examples I through IX were concerned with the hypochlorite bleaching stage of the pulp process with the Example I being a control or reference example. The Examples X through XV were concerned with the chlorination stage of bleaching and the Examples X and XIV were reference or control examples. By running a series of examples such as the Examples I, X and XIV, the brightness of the paper produced from the control or reference pulp can be readily compared with the brightness produced from the pulp which employed the novel teachings of the present in vention. That is to say, in the Examples I, X and XIV, no aluminum chloride was added to any of the bleaching stages.

The trivalent cation which was used to speed up the oxidation bleaching reaction of hypochlorites and chlorine on the kraft pulp was the aluminum cation and its source compound was aluminum chloride. The source compound should be composed of a trivalent cation and monovalent anions. Although other trivalent cations are available, the use of aluminum chloride as a source of trivalent cations was prompted by considerations of economics, availability, solubility, and the absence of color. Aluminum chloride is readily available and fairly economical to produce or purchase. It is white in color and soluble up to approximately 70 grams per milliliters of cold water. Unlike many compositions, mixtures, compounds or elements employed for bleaching, the use of aluminum chloride is outstanding in that it introduces no color of its own. In addition, the other considerations of economics, availability and solubility prompt its adoption in the practice of the invention.

For a further and more complete understanding of the invention, the following illustrative but non-limiJng examples of the chlorine and hypochlorite bleaching opthe stages of bleaching (hypochlorite or chlorination) or in both stages.

Example I In this example, a reference or control experiment was run (no aluminum chloride was added to the hypochlorite bleaching stage of the pulp)'so that the paper produced from this pulp could be compared in brightness to the paper produced from the pulp in which the aluminum chloride was added. The pulp was produced according to the kraft or sulfate process. As defined earlier, a kraft pulp is a pulp which is obtained from the reaction of either coniferous or deciduous wood with a cooking liquor which has as its principal active constituents sodium hydroxide and sodium sulfide. The reaction was carried out in a closed pressure vessel at a temperature of about 340 F. After the cooking or digesion stage, the pulp was washed to remove residual spent cooking liquor and dissolved wood components. The remaining kraft pulp consisted primarily of cellulose and hemicellulose along with small amounts of lignin, wood resins, meal salts, and non-cellulose carbohydrate components. Typically, the pulp had a dark brown color.

The pulp was produced from hardwood and it was determined that a pre-bleach of the pulp with sodium hypochlorite (a hypochlorite bleach stage) under alkaline conditions prior to the chlorination stage was desirable. In Example la 8.0% by weight of chlorine as sodium hypochlorite along with 1.0% sodium hydroxide as the caustic for a buffer was added to the washed pulp having a consistency, O.D. (oven dry) of 3.0%. Example lb except that 0.05% by weight of dry pulp of aluminum chloride as aluminum trivalent cation was added. The temperature was maintained at 95 F. for 40 minutes. In the control or reference Example Ia, 42.9% of the chlorine was consumed and a brightness (all brightnesses will be that as determined by a GE. apparatus) of 59.0 was obtained. In Example lb which included aluminum chloride, 60.0% of the chlorine was consumed and a brightness of the paper produced from this pulp was determined to be 69.6, an increase of 6.6 brightness points over the Example Ia which did not use or employ the aluminum chloride.

Example II The conditions for Example Ila and Ilb are substantially identical to those of Example Ia and Example Ib except that the time was increased to 60 minu es. The chlorine consumed by the control or reference Example 110 was 45.4% which yielded a brightness of 62.1. The chlorine consumed by the Example Ilb which included the aluminum chloride, was 61.8% and a final brightness of 73.8 was obtained. A difference in brightness of 11.7 brightness points was obtained in Example IIb over that of Ila which did not contain aluminum chloride.

Example III The present Example 111a and Example IIIb are substantially similar to the foregoing examples except that the time is increased to 120 minutes and the temperature is increased to 110 F. The control or reference Example IIIa showed that 57.1% of the chlorine was consumed which yielded a brightness of 69.0. In the Example IIIb, to which aluminum chloride in the amount of 0.05 was added, 83.8% of the chlorine was consumed and a brightness of 79.0 was obtained. Thus, a 10.0 brightness point increase was achieved through the use of the aluminum chloride.

Example IV Examples IVa and IVb are substantially similar to Examples IIIa and IIIb, respectively, except that the time is increased to 180 minutes. The temperature is maintained at F. In the control or reference sample identified as Example IVa, 70.1% of the chlorine was consumed which yielded a brightness of 72.7. On the other hand, the Example IVb which contained 0.05% of aluminum chloride, 92.7% of the chlorine was consumed which yielded a brightness of 80.0, a 7.3 brightness point in crease over Example IVa.

Example V In this example, the time is extended to 220 minutes so that Example Va and Example Vb are substantially identical to Example IVa and Example IVb, respectively except for the increase in time of 40 minutes. The control or reference Example Va was found to have con-- sumed 75.5% of the chlorine so as to yield a brightness of 76.3. The Example Vb containing 0.05% aluminum chloride consumed 99.5% of the chlorine and yielded a brightness of 80.2, an increase of approximately 3.9 brightness points over the Example Va which did not contain any aluminum chloride.

Example VI In this example, the aluminum chloride was reduced so as to establish a lower limit for the quantity of aluminum cation required to speed up the reaction of the by pochlorite bleaching stage. Accordingly, to the kraft pulp substantially identical to that employed in the fore going examples, was added 0.004% aluminum chloride, 8.0% sodium hypochlorite, 1.0% sodium hydroxide, with a consistency CD. of 3.0%. The temperature was maintained at approximately 95 F. for 60 minutes. The brightness of the pulp is found to be approximately 73.0 which compares favorably with a brightness of 73.8 when the aluminum cation was present in the amount of 0.05% by weight of dry pulp. Thus, a reduction in the amount of aluminum chloride from 0.05% to 0.004% does not significantly affect the results.

Example VII In this example, an experiment is run to determine an upper limit for the quantity of aluminum chloride required to speed up the reaction. The conditions and quantities are substantially similar to Example VI except that the aluminum chloride is added in the amount. of 0.16% by weight of dry pulp to the pulp. After a determination, the brightness of the pulp is found to be approximately 74.0 points which, when compared to a brightness of 73.8 points when using 0.05% aluminum chloride, suggest that the greater quantity of aluminum chloride had a little effect upon the final brightness of. the pulp.

Example VIII In this example, the temperature is reduced along with a reduction in the amount of aluminum chloride employed over that set forth, for example, in Example lb. More specifically, the temperature of the pulp was main tained at 90 F., for 60 minutes and the aluminum chloride was added in*the amount of 0.04%. The brightness of the pulp at the end of 60 minutes bleaching was found to be approximately 72.0 points which when compared to a brightness of 73.8 points in Example Ilb, shows that a reduction of the temperature by 5 F. does have some effect upon the reaction but not so great an effect so as to substantially alter the results obtained.

Example D( In this example, an experiment is run to determine an upper limit for the temperature of the reaction. More specifically, the conditions in this example are substantially identical to those of Example VIII except that the temperature is maintained at F. for 60 minutes. The brightness of the pulp is found to be approximately 75.5 points which when compared, for example, to Example IIb, shows a brightness increase of approximately 7 2 points for an increase in temperature of approximately 20.

The foregoing examples represent the results obtained with the addition of aluminum chloride (except in the control or reference examples) to the pulp in a hypochlorite bleaching stage. The results of Example In through Example Vb are tabulated below to as to permit easy and convenient comparison of the beneficial results obtained in the practice of the invention:

TAB ULATION OF EXAMPLES Ia THROUGH Vb 0.05% Percent Al (.1 Time Temp. C1 Brlghness Example added (in min.) (degFahJ consumed (G.E.)

The following examples are experiments X through XV that were run to determine the effect of the addition of aluminum chloride to the chlorination bleaching stage of a kraft pulp produced in accordance with the earlier examples. The pulp was cooked, washed, screened, cleaned and bleached in the first two stages in the usual manner and the aluminum cation was added to a third stage chlorination bleach. The specifics are set forth in the following examples:

Example X A kraft pulp was prepared in the manner set forth in the earlier examples. After the cooking or digestion stage, the pulp was washed to remove the residual spent cooking liquor and dissolved wood components. Before the pulp was subjected to a chlorination stage in which the aluminum cation was added, the pulp was bleached at a consistency CD. of 10 with hypochlorite at a temperature of 105 F. for 25 minutes. Thereafter, the pulp was washed and then subjected to a chlorination stage under immediate study. The consistency CD. was 3, the temperature 90 F., and 1.5% chlorine by weight was added. No aluminum cation was added to this example so that this example will serve as a reference or control. At the end of minutes, 35.2% of the total chlorine was consumed while at the end of 40 minutes, 38.5% of the total chlorine was consumed. The pH of chlorination was 2.9. After subsequent washing, extraction with sodium hydroxide, washing, and again bleaching, the brightness was 80.4 points (G.E.).

Example XI The conditions for this example are substantially identical to those of Example X except that 3800 gallons of 1A seal water was added per ton of pulp along with 0.04% of aluminum chloride. 1A seal water is water from the kraft chlorination washers. The percentage of chlorine consumed at the end of 20 minutes was found to be 73.2% while the percentage of chlorine consumed at the end of 40 minutes was found to be 87.4%. The pH of chlorination was 2.4. After the subsequent washing, extraction, washing and bleaching, the brightness was determined to be 84.4 points or an increase of approximately 4.0 points over the control or reference sample of Example X.

Example XII In this example, the first bleaching stage referred to in Example X was conducted at a slightly higher temperature and a greater time. More specifically, the temperature was 110 F. and the time period was 30 minutes. The conditions for chlorination stage were substantially identical to those of Example XI except that 0.02% aluminum chloride was added instead of 0.04% as set forth in Example XI. The percentage of chlorine consumed at the end of 20 minutes was found to be 70.9% while the percentage of chlorine consumed at the end of 40 minutes was found to be 83.2%. The pH of chlorination was 1.80. After subsequent washing, extraction, washing, and bleaching, the brightness was determined to be 87.0 points. When compared to Example X, the increase in brightness was approximately 6.6 points and when compared to Example XI, the brightness increase was approximately 2.6 points. The diminished quantity of aluminum chloride appears to have little effect while the increase in brightness may be attributable to the increase in temperature of the first bleaching stage.

Example XIII The conditions for this example were substantially identical to those of Example XII except that 0.04% aluminum chloride was added in place of the 0.02% of Example XII. At the end of 20 minutes, 72.04% of the chlorine was consumed while at the end of 40 minutes 84.7% of the chlorine was consumed. After the usual washing, extraction, etc. stages, the brightness was determined to be 87.5. A comparison of this brightness with the brightness obtained in Example XII, shows that only a 0.5 point brightness increase was obtained by doubling the amount of aluminum chloride.

Example XIV This example was run as a control or reference experiment for comparison with subsequent Example XV. The first bleaching stage (the bleaching stage prior to the bleaching stage where in the addition of aluminum cation is considered) was at a consistency CD. of 10, a temperature of 105 F., a time period of 25 minutes, with 4% bleach as chlorine added. The pH range was 8.0 to 10.5. After washing, the chlorination stage to be considered in detail at a consistency CD. of 3, a temperature of 81 F. and 1.5% chlorine added on a pulp basis. After 40 minutes, 54.2% of the chlorine was consumed and the pH of chlorination was 4.4. A brightness of 80.6 was obtained after the usual washing, extraction, etc. set forth. in the Example X and subsequent examples. 3

Example XV This example was substantially identical to Example XIV except that the temperature was 82 F., 0.03% aluminum chloride was added and 3800 gallons of 1A seal water per ton of pulp was mixed with this chlorina tion stage. The percentage of chlorine consumed at the end of 40 minutes was found to be 77.1%, a gain of ap proximately 23% over that of Example XIV wherein no aluminum chloride was employed. After the washing, extraction, etc. stages, the brightness was found to be 85.7 which was a gain of over 5 points from that of Example XIV.

In order to more clearly compare the results obtained in Examples X through XV, the following tabulation is set forth:

TABULATION OF EXAMPLES X THROUGH XV Temp. 01

Al Cl; Temp. previous Per cent added, Time (deg. bleach Cl; con- Bright Example percent (in min Fah.) stage snmed ness X.-. 0.00 40 38. 5 80. 4 0. 05 40 90 105 87. 4 84. l 0.02 40 90 83. 2 87. 0 0. 04 40 90 110 84. 7 87, 5 0.00 40 81 105 54. 2 80 n X" 0.03 40 82 105 77. 1 85. 7

A comparison of the Examples XI, XII and X11] with the control Example X, clearly demonstrates that more than twice the amount of chlorine was consumed so that, for the same time period, a brightness increase of up to 7 points was obtained. Similarly, a comparison of the Example XV with the control Example XIV, clearly illustrates an increase in the quantity of chlorine consumed and a resulting increase in brightness of over points.

Thus, it has been clearly shown and illustrated that the addition of an aluminum cation, such as the cation resulting from the ionization of aluminum chloride, produces significant increases in brightness. The invention clearly sets forth a great efficiency in the practice of the invention whether it is practiced with the hypochlorite bleaching stage, the chlorination bleaching stage, or even with both stages.

Typically, an unbleached kraft pulp has a dull-dark brown color. This color is not easily dyed to other colors, nor does it give a pleasing contrast to printing ink colors. Furthermore, high grade white papers would not be produced from a pulp having such a dark color. The two alternative methods by which the color of the unbleached kraft pulp may be lightened and brightened are by more severe cooking or digestion and by bleaching. To improve the color by increasing the severity of the cooking stage is extremely costly and prior to the discovery of the condition set forth in this invention, color improvement through bleaching was prohibitively costly. However, the present invention has overcome the high cost factor by employing a trivalent cation to speed the oxidation bleaching reaction of chlorine and of hypochlorite on kraft and other pulps. The source of the trivalent cation is aluminum chloride. Although other trivalent cations are available the use of aluminum chloride as a source of trivalent cation was prompted by considerations of economics, availability, solubility, and the absence of color.

What is claimed is:

1. A method for improving the color characteristics of paper products in a multistage pulp bleaching process which comprises the addition of at least 0.004% by weight of dry pulp of an aluminum chloride in an aqueous medium to a bleaching stage selected from the group consisting of hypochlorite oxidation and chlorine oxidation.

2. The process as defined in claim 1 wherein the bleaching stage is a hypochlorite oxidation bleaching reaction and the compound is aluminum chloride.

3. The process as defined in claim 1 wherein the bleaching stage is a chlorine oxidation bleaching reaction and the compound is aluminum chloride.

4. A process for improving the brightness characteristic of paper which comprises subjecting a pulp to a sodium hypochlorite bleaching stage at a temperature in the range of 90 F. to 115 F. and adding at least 0.004% by weight of dry pulp of an aluminum chloride in an aqueous medium to the pulp.

5. A process for improving the brightness characteristics of paper as defined in claim 4 which comprises adding 0.004% to 0.16% by weight of dry pulp of an aluminum chloride in an aqueous medium to the pulp, and maintaining the temperature for a period of from 40 minutes to 220 minutes.

6. A process for improving the brightness characteristics of paper which comprises subjecting a pulp to a 10 chlorine bleaching stage at a temperature in the range of 60 F. to 100 F. and adding at least 0.01% by weight of dry pulp of an aluminum chloride in an aqueous medium to the pulp.

7. A process for improving the brightness characteris'ics of paper as defined in claim 6 which comprises adding 0.01% to 0.10% by weight of dry pulp of an alumi num chloride in an aqueous medium to the pulp, and maintaining the temperature for a period of from 20 minutes to 60 minutes.

8. A process for improving the brightness character istics of pulp which comprises subjecting the pulp to multistage bleaching stages including washing and caustic extraction stages, said bleaching stages 'being a hypochlorite stage followed by a chlorine stage or vice versa, said hypochlorite stage including the steps of maintaining the temperature of the pulp in a range of F. to 115 F., adding an efiective amount of an aluminum chloride in an aqueous medium in a weight range of 0.004% to 0.16% by weight of dry pulp to the pulp, and maintaintaining the temperature of the pulp for a period of from 50 minutes to 220 minutes, said chlorine stage including the steps of maintaining the temperature of the pulp in a range of 60 F. to F., adding 0.01% to 0.10% by weight of said aluminum compound to the pulp, and maintaining the temperature for a period of from 20 minutes to 60 minutes, all of said stages including the addition of an effective quantity of water to insure ionization of said aluminum compound.

9. A process for improving the brightness characteristics of pulp which comprises subjecting the pulp to multistage bleaching stages including washing and caustic extraction stages, said bleaching stages being a hypochlorite stage followed by a chlorine stage or vice versa, said hypochlorite stage including the steps of maintaining the temperature of the pulp at about 90 F., adding about 0.04% by weight of dry pulp of aluminum chloride to the pulp, and maintaining the temperature of the pulp for about 90 minutes, said chlorine stage including the steps of maintaining the temperature of the pulp of about 80 F., adding about 0.04% 'by weight of aluminum chloride to the pulp, and maintaining the temperature of the pulp for about 40 minutes, all of said stages including the addition of an effective quantity of water to insure ionization of said aluminum chloride.

References Cited UNITED STATES PATENTS 312,525 2/1885 Souders 16279 2,194,956 3/1940 Taylor 162-87 X 2,676,885 4/1954 Hamburg 162-87 X HOWARD R. CAINE, Primary Examiner US. Cl. X.R. 162-88; 25299 

